Converts between diopters and focal length

Telescope

Diopter
Focal length inches millimeters

Eyepiece

Diopter
Focal length inches millimeters

Result

Notes:

The power of a lens or mirror can be given by its diopter. When two lenses are close together their combined power is the sum of the two diopters - easy!
When the lenses are far apart such that the telescope lens forms a real image that is projected into the eye by the eyepiece lens, then the eyepiece diopter is divided by the telescope diopter.

by Mel Bartels